The EDM process makes use of electroerosive power pulses applied between a workpiece and a tool electrode spacedly juxtaposed across a machining gap filled with a dielectric liquid (e.g. Kerosene or distilled water) which also serves to carry away from the machining gap the detritus of the electrical discharge machining process.
The tool electrode is generally formed with the desired configuration of the cavity or shape complementarily desired in the workpiece. A train of power pulses are then formed to create localized, discrete and randomly dispersed stock removal discharges which produce cumulatively overlapping craters in the workpiece surface; the total surface juxtaposed with the tool electrode is thus machined uniformly over the parts thereof confronting the tool electrode and receives a configuration conforming to the shape of the tool electrode. In traveling-wire or wire-cut electrical discharge machining (TW-EDM), the tool electrode is constituted by a continuous, axially-traveling elongate wire-like electrode and a two- or three-dimensional relative displacement between the wire and the workpiece yields a desired shaped configuration in or on the workpiece.
During the machining operation, small metallic or conductive chips or particles removed from the electrode surfaces as well as other discharge products such as tar and gases are carried away by the liquid dielectric which floods the gap and is generally circulated therethrough while the tool electrode is advanced relative to the workpiece by a servo system designed to maintain a predetermined gap spacing substantially constant or to approach the desired gap spacing as accurately as possible. The servo arrangement may also function to respond to gap short-circuiting and arcing conditions to retract the electrode relative to the workpiece thereby removing such conditions.
It can also be stated that in an EDM process, electric energy is furnished from the power supply in the form of discrete electrical pulses across the machining gap filled with a liquid dielectric to effect a succession of electrical discharges between the tool electrode and the workpiece to remove stock from the latter. Each individual discharge strikes that area of the workpiece juxtaposed with the tool electrode on one minute localized zone or another, the zone being impulsively melted and/or vaporized and mechanically dislodged from the workpiece area by the impulsive discharge pressure. Successive and repetitive discharges are used to sweep the localized stock dislodgment or removal action over the entire workpiece area and result in the formation of cumulatively over-lapped discharge craters thereon. As stock removal proceeds, the tool electrode is advanced relatively towards the workpiece by servo feed means adapted to maintain the machining gap spacing substantially constant and thereby to allow stock removal discharges to be successively created. The tool electrode in sinking-type EDM is generally formed with the desired configuration of the cavity or shape complementarily desired in the workpiece. Thus, the total surface eventually juxtaposed with the tool is machined over those portions thereof which confront the tool electrode and receives a configuration conforming to the shape of the tool electrode. In travelling-wire or wire-cut EDM in which the tool electrode is formed by a continuous, axially traveling elongate wire-like electrode or in scanning-type EDM using a rod or the like electrode having a relatively simple machining contour, a two- or three- dimensional relative displacement is effected between the electrode and the workpiece to yield a desired shaped configuration in or on the workpiece corresponding to the path of the relative displacement. The contamination of the machining gap region with chips, tar and gases produced by machining discharges may be eliminated by continuously or intermittently flushing the gap with a fresh machining fluid and/or intermittently or cyclically retracting the tool electrode away from the workpiece to allow the fresh machining medium to be pumped into the machining gap and the machining contaminants to be carried away from the latter.
Parameters of individual and successive electrical discharges or machining current pulses, especially pulse on-time .tau.on and peak current Ip are, for a given combination of electrode materials and other machining settings, determinative of stock removal characteristics per single pulse delivery and hence of critical machining results, i.e. removal rate, surface roughness and relative electrode wear and, therefore, must be adjusted, in conjunction with pulse off-time, to establish a particular machining condition suitable to yield the desired machining results. These parameters are adjusted individually at pulse source circuitry in the power supply or a pulse generator which is, preferably, of solid-state or semiconductor switching type.
The present inventor has observed that in a conventional EDM arrangement, no matter how accurate the setting of these parameters is done at the pulse source in the power supply, the pulse becomes distorted while being generated and transmited to the gap through the gap discharge circuit. It has been observed that the distortion is brought about due to stray capacitances inherently distributed in the gap discharge circuitry, which circuitry includes the machining gap between the tool electrode and the workpiece separated by the dielectric liquid, a semiconductor power switching network for pulsing a DC source, various leads in the power supply, cables connecting the power switch to the gap site, and conductors leading from the power cables for directly energizing the tool electrode and workpiece, and also to some extent environmental circuit units for mechanical arrangements. Heretofore, little care has been exercized with the respect to these stray capacitances contained in the gap discharge circuit. The present inventor has now noted that these stray capacitances have significant influences, which cannot be neglected, on the characteristics of a discharge pulse which eventually develops across the machining gap.
Specifically, stray capacitances are present generally across the machining gap between the tool electrode and the workpiece separated by the dielectric liquid, at the interfaces between the emitter and base of a semiconductor element in the switch unit for pulsing a DC power supply to produce the power pulses, and between the parallel conductors in a printed circuit board, lead cables, and at the portions of insulators for the electrode supporting head and for the workpiece support. In total the stray capacitances have been found to amount to 100 to 1000 picofarads or more in a conventional EDM arrangement using, as a dielectric liquid, kerosene having a specific resistivity of 10.sup.10 to 10.sup.12 ohm-cm for machining a workpiece area in excess of 4 cm.sup.2. It has now been discovered that the presence of stray capacitances of such magnitude is a significant cause of, among other things, a tendency towards gap short-circuiting and arcing, development of machining instability, inability to raise the removal rate and excessive electrode wear, and the unsatisfactory machined surface quality. Still more important, the distortion of the discharge-current waveform cannot be neglected where narrow or extremely narrow power pulses are to be employed to seek to achieve a fine or super-fine EDM machined surface quality. Thus, there has hitherto been a practical limit in improving the surface finish of an EDM machined surface. Furthermore, the relative electrode wear may then amount to as high as 100% and render it impossible to achieve a desired finishing EDM operation.